It's unclear who threw the brick that hit and killed a 70-year-old man in Hong Kong.

"The patient's situation continuously deteriorated. He passed away" late on Thursday, a spokesperson for Hong Kong's Prince of Wales hospital said in a statement.

It was not immediately clear who threw the brick in an incident which was filmed and widely shared on social media.

He is the second person in less than a week to die in protest-linked incidents.

Alex Chow, 22, died on 8 November from head injuries sustained during a fall as police skirmished with demonstrators inside a car park.

The latest death comes as China's President Xi Jinping warned that protests in Hong Kong were threatening the "one country, two systems" principle governing the semi-autonomous city.

Hong Kong has been ruled by a unique system guaranteeing greater freedoms than on the mainland since its handover from British rule to China in 1997.

But protests, which began against a now-shelved extradition bill to China, have spiralled into wider calls for democracy and police accountability as violence and demonstrations roil the city, challenging Beijing's authority.   

For a fourth straight day on Thursday, protesters caused widespread disruption with barricades and rallies, as the police drafted in reinforcements and the government denied rumours of an imminent curfew.

The five-month crisis has entered a new phase in recent days with hardcore protesters embarking on a campaign to "blossom everywhere" across the city in a bid to stretch police resources.

The protests, fuelled by fears that the territory's China-backed government is encroaching on the city's freedoms, are backlit by fears China may send in its troops to squash the movement. 

n rare comments on the violence, Xi repeated Beijing's unwavering support for the Hong Kong government and police, warning recent actions by protesters have "seriously challenged the baseline principle of 'one country, two systems'."

Speaking at a summit in the Brazilian capital of Brasilia, he said "stopping violence and controlling chaos while restoring order is currently Hong Kong's most urgent task," in comments reported across Chinese state media.

Hong Kong has been bitterly divided by the increasing protests and violence.

Roads, schools closed:

The unrest has caused multiple injuries but relatively few deaths despite the barrage of police rubber bullets and occasional live fire, as well as petrol bombs and bricks hurled by protesters.

On Thursday, key arterial roads were blocked, many train services suspended and schools closed, while lunchtime rallies took place in the business district as protesters occupied universities.

With the protesters showing no signs of relenting, the nearly 30,000-strong city police force announced it was drafting in 100 prison guards and looking for other reinforcements.

"The ongoing riots... with their massive scale, simultaneous occurrence in various districts and grave severity of violence, make it necessary to strengthen the support for the police's front-line officers," a police spokesman said.

While there was no suggestion in China on Thursday of the military being deployed, one of the most prominent state-run media outlets, the nationalistic Global Times, raised tensions with a report that a curfew was on the cards.

In an English post on its Twitter account, the Global Times said the Hong Kong government was looking to implement a weekend curfew, echoing unsubstantiated rumours online.

However the Global Times quickly deleted the tweet, and Hong Kong's government said the rumours were "totally unfounded".